Wind turbines typically include a rotor with large blades driven by the wind. The blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy is typically transferred via drivetrain to a generator, which then converts the energy into electrical power.
Most modern wind turbines control power output by pitching the blades relative to the wind. Thus, each blade is mounted to a hub by a blade bearing that allows relative movement between the blade and the hub. The blades are rotated about their longitudinal axis by a pitch system that includes one or more electrical drives (e.g., electrical motors) or hydraulic drives (e.g., hydraulic actuators).
Pitch control places significant demands on the blade bearings because they are subjected to a high level of activity. The small, cyclical movements that characterize pitch control can reduce fatigue life. As a result, various approaches have been taken to increase the load capacity of blade bearings.
One of the most conventional approaches is to include two or more rows of rolling elements (e.g., balls or rollers). Two-row ball bearings are often used as blade bearings, and three-row ball bearings and three-row roller bearings have drawn increased interest in recent years as the size (and loads) of wind turbines has grown.
The additional rows of rolling elements, however, may still not be sufficient to meet design loads. The blade bearings may still experience distortions that affect their capacity. Furthermore, the additional rows can lead to problems of load distribution. One option to address these challenges is to increase the size and weight of the blade bearings, but this can lead to a significant increase in costs. Not only for the blade bearings themselves, but also for the blade and hub which must increase in size to accommodate the blade bearings.
Another option is to stiffen the rings of the blade bearings with an additional support structure, such as a plate. Such plates are mounted on one or both sides of a bearing ring, particularly the inner ring. Although reinforcing the inner ring on both sides may keep radial distortions low, it can also limit deflections in the axial direction. This may lead to high contact angles for the rolling elements and poor load distribution when the outer ring deflects in the axial direction (effectively tilting relative to the inner bearing ring).